The proposal aims to investigate how stress is internalized to affect cognitive functioning and increase the risk for Major Depressive Disorder (MOD). Dysfunctional stress reactivity may be a risk factor for MOD, yet mechanisms underlying this process remain unexplained. Both stress and MOD have been identified as compromising factors on "higher level" cognitive control systems and "lower level" sub-cortical systems. Similarly, both stress and MDD may alter an individual's sensitivity to reward and punishment (reinforcement). This proposal aims to identify the roles of frontal cortical and striatal systems in stress and MDD during reinforcement learning. Two participant groups will be tested: a non-depressed (n=75) and a depressed (n=25) group. All participants will first complete a probabilistic reward learning task. Following that task, the depressed group and a randomly assigned two thirds of the non-depressed group will complete a similar task under social stress. The other non-depressed participants will complete the same task under standard conditions. Electroencephalographic (EEC) recordings will be obtained during the tasks, which will allow objective measurement of neural activities reflective of reinforcement learning. Facets of acute stress reactivity (emotional and cortisol reactivity) will be investigated as moderators of the stress-learning link. This design will allow consideration of: 1) how specific neural systems become functionally compromised during high stress reactivity and 2) whether these same systems are compromised in depression. PUBLIC HEALTH RELEVANCE: An understanding of the specific way that stress alters reinforcement sensitivity may reveal distinct neural mechanisms mediating the translation of prolonged stress into ongoing affective distress. The combined use of physiological and computational models to understand how stress, emotion, and cognitive processes contribute to mental disorders such as major depression is innovative, and in line with the high priority statements of the NIMH regarding combined approaches. Future directions may reveal objective risk factors for dysfunctional stress reactivity, providing tailored approaches of diagnosis and treatment for MDD. [unreadable] [unreadable] [unreadable]